Brownian Motion in Viscoelastic Media

Abstract

The motivation of this experimental research project is to gain a better understanding of the potential connection between the Brownian motion of a probe particle and the dynamic response of the suspending media. Specifically, the objective is to gain an understanding of the effects of longitudinal and transverse modes on the extracted viscoelastic parameters. This is achieved through tracer microrheology/mechanical rheometry study of three representative complex fluid systems. The chosen systems are,(1) aqueous poly ethylene oxide (PEO) solution, (2) cetyltrimethylammonium bromide/ sodium salicylate worm like micelle solutions and (3) a poly ethylene oxide-poly propylene oxide-polyethylene oxide (PEO-PPO-PEO) triblock copolymer, Pluronic™ F108 in an aqueous solution.

The PEO system exhibits a flat plateau followed by a diffusive escape at concentrations above 1wt% and exhibits increasing elasticity with increasing concentration. The PEO system tracer microrheology and mechanical rheometry measurements agree very well. A notable result for this system is that the longitudinal dynamical modes and structural features do not significantly affect the extracted viscoelastic data, as revealed through a sphere size sweep on the system.

The CTAB/NaSal system exhibits polyelectrolyte like behavior in that the static and dynamic properties in these solutions exhibit a complex dependence on ionization and electrostatic screening effects. Tracer microrheology exhibits deviations from the mechanical rheometry measurements, as manifested through the creep compliance measurements. Sphere size sweeps and tracking of the deviation with the bulk longitudinal elastic modulus tends to indicate the possible role of the longitudinal dynamical modes on the extracted parameters.

Tracer microrheology measurements on the Pluronic™ F108 system exhibits widely varying behavior as the system passes from an isotropic micellar solution to an FCC soft crystal, illustrating the sensitivity of the tracer microrheology technique to changes in microstructure and dynamics. The tracer microrheology complex viscoelastic moduli G* is seen to underestimate the mechanical rheometry G* especially at high concentrations. Sphere size sweeps on the system indicates that the significance of the longitudinal dynamical modes in affecting the extracted viscoelastic properties is very sensitive to the intermicellar separation and probe size.